Pulsed Laser Deposition for Coating Applications - DOC

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					                       UV excimer lasers for smart materials and nanostructures

                                      B. Fechner, R. Pätzel, R. Delmdahl
                         Coherent GmbH, Hans-Böckler-Str. 12, D-37079 Göttingen, Germany
                                     E-mail: burkhard.fechner@coherent.com


                                                       Abstract

From medicine through consumer electronics, device manufacturers face market pressure to increase
miniaturization while increasing device functionality and hence complexity. As a result, many industries are
turning to laser micromachining as a manufacturing solution to meet these needs. Excimer lasers have already
proven particularly well-suited to these micromachining applications. Ongoing progress in material research and
processing industry is fueled to a large extent by the technique of pulsed laser deposition (PLD). With this
powerful and versatile approach, multi-component target materials can be ablated and deposited onto a substrate
to form stoichiometric layers which exhibit the desired properties. Monitoring of growth parameters such as
thickness and surface roughness is frequently in-situ monitored via electron diffraction or other diagnostic tools.
Both quality and longevity of the microstructures acting e.g. as sensors, actuators, bioreactors or information
transmitters strongly depend on the degree of accuracy achieved in the manufacturing process.


                                               1.       Introduction

Pulsed Laser Deposition (PLD) as a physical vapour deposition technique for coating development and material
screening opens up nearly unlimited pathways to functional coatings by means of rapid protocoating.
Prerequisites for a successful rapid protocoating are well-conceived ablation systems and lasers enabling
efficient, development of thin film coatings for medical device manufacturing, mechanical engineering,
microsystems technology or optics on a short timescale. In the PLD technique a high pulse energy laser beam,
preferably the rectangular profile of a short wavelength excimer laser at 248 or 193 nm, is demagnified on the
target material which is to be deposited. Due to the short wavelength of the pulsed excimer light (20ns) and the
resulting small penetration depth, the absorption takes place selectively in a limited volume near the surface
leading to fast heating and explosive evaporation1. This non–thermal equilibration mechanism is the basis for
depositing multi-component substrate materials controlling stoichiometry and crystal properties during thin film
growth.
The high energy photons of the excimer laser allow virtually all target materials to be deposited such as oxides,
nitrides, and carbides for isolators, metals, complex ceramics, and polymers for semiconductors. The flexibility
in view of the employed materials which can be varied during the deposition process allowing straightforward
tayloring of multicoatings has rendered PLD an established and productive technology for coating and material
development2.


                                        2.     Pulsed Laser Deposition

                                             2.1       Ablation source

Uniform pulse energy, at both low repetition rates
and in burst operation, is among the most critical
laser output parameters for PLD. A constant,
uniform pulse energy produces consistent
deposition parameters, resulting in homogeneous
films and a repeatable process. High laser pulse
energy provides several benefits for PLD. First, it
enhances the deposition rate of target materials.
Depending on laser pulse energy several microns
per minute are achievable. Next, it enables a larger
area on the target to be ablated at a given fluence.
This area enlargement increases the deposition rate
and reduces the plume angle, resulting in higher
deposition efficiency. Finally, higher photon             Fig.1 Pulse energy and energy stability of COMPexPro as a
energies as provided by excimer lasers at                 function of operation voltage at 248 nm and 10Hz.
wavelengths of 193 nm and 157 nm provide an even larger process window, allowing consistent, successful
material ablation well above the ablation threshold also for transparent polymers and hard target samples3. Even
compact excimer lasers provide high pulse energies between 200mJ and 500mJ with excellent pulse-to-pulse
stability of typically 0.5%, 1 sigma.


                                             2.2   Vacuum system

In order to generate smart material layers most
effectively next to the ablation light source
which is preferably a short wavelength excimer
laser a sophisticated vacuum system is the key to
success. Its essential components are the vacuum
chamber containing heated substrate holder,
target holder and UV optical elements for
demagnifying the laser beam to the required on-
target energy density of typically 1-5 J/cm2.
Both a constant deposition rate and homo-
geneous thin film properties over a large thin
film area are provided by the exceptional pulse-
to-pulse stability and beam homogeneity of
advanced high-pulse energy lasers.
Fully automated vacuum systems with up to 6
inch diameter substrates enable efficient and
reproducible thin film development for scientific     Fig.2 Target holder in an advanced PLD vacuum system,
as well as industrial research facilities. Rotatable  consisting of six rotatable targets.
revolvers, as shown in figure 2, allow to variably
deposit up to 6 different target materials. The individual targets generally consist of small pellets offering high
flexibility and reducing target costs to a minimum.


                            2.3    Coating capabilities of Pulsed Laser Deposition

Of particular interest both in mechanical and optical engineering are coatings combining hydrophobic
functionality with a high degree of trans-parency in a thin layer as provided by poly-tetrafluoroethylene (PTFE).
This material cannot be deposited other than with pulsed laser deposition and demonstrates the fle-xibility of
PLD. Thin PTFE layers of a thickness of above 100 nm significantly increase the contact angle on a given
substrate to 110° as is shown in figure 3 for glass substrate and at the same time provide a transmission of >98 %
as useful for e.g. self-cleaning surfaces.

In medical device technology PLD deposited coatings lend the required biocompatibility to novel implants. As in
the case of stents many devices cannot be made from biocompatible materials such as titanium directly but need
to be chosen in view
of their mechanical
properties supporting
high tensile stress
during expansion in
the blood vessel. The
appropriate layer ma-
terial deposited with
PLD exhibits high
adherence also on
the usually four
times expanded stent
mate-rial which is Fig.3 Water droplet on a glass surface before (left picture) and after (right picture)
the prerequisite for coating with a PTFE thin film (Axyntec GmbH).
its biocom-patibility.
In figure 4, a biocompatible metal oxide target has been used for pulsed laser deposition with excellent thin film
homogeneity and strength. Deposition time for a 150 nm film on a 20 mm long stent is in the range of minutes.




Fig.4 Expanded stent coated with a thin metal oxide layer (left). The enlarged view (right) gives evidence for
the high thin film quality (Axyntec GmbH).




                                              3.     Microfluidics

Highly miniaturized devices in biomedicine include relatively simple products, such as micro-arrays used in the
pharmaceutical industry for high throughput drug discovery, and more complex microfluidic devices. These lab-
on-chip devices are widely used in genomics and
proteomics, and will soon enable the miniaturization and
automation of analytical testing. Typically resembling
microscope slides, lab-on-chip devices are fabricated in
optically transparent materials, such as pyrex glass and
PMMA, to enable analysis using some type of modified
microscope setup. Unfortunately, it is difficult to create
microscopic channels, grooves, holes and bridges in these
materials by traditional methods, particularly in the case of
glass. But excimer micromachining can create these
features with the desired resolution and without any
microcracking or other problems (see figure). The 248 nm
output wavelength is commonly used for polymers and the
193 nm wavelength is mostly used for glass and quartz
machining.
In addition, many lab-on-chip systems require electrical
contacts, to enable processes such as electrophoresis. The
excimer can also be used to fabricate these electrodes in the
back-side of the lab-on-chip. Each electrode is produced by
ablating a small through hole at the required location.
These often have a circular cross-section with typical
diameters of a few tens of microns or less. Other shapes
can be created with an appropriate photomask, which also                                                             Formatted: Font: 10 pt, Bold
allows all the electrodes to be drilled in a single step. After
                                                                                                                     Formatted: Font: 10 pt, Italic
laser-drilling, the holes are completely filled with metal in a
                                                                Fig.5 Microfluidic structure and sensors             Formatted: Space Before: 6 pt, Line spacing:
vapor deposition or pulsed laser deposition process, forming
                                                                (Bartels Mikrotechnik, GmbH)                          single
both a liquid-tight seal and a through electrode (see fig. 5).
                                                                                                                     Formatted: Font: 10 pt, Italic
                                                                                                                     Formatted: Font: 10 pt, Italic
                                                                                                                     Formatted: Font: 10 pt, Italic
                                                                                                                     Formatted: Font: 10 pt, Italic
                                      4.     Direct Patterning of Circuits

There is growing demand for low unit cost, miniaturized electrical circuits for applications such disposable
medical sensors and radio frequency identifiers (RFID).
In this application, the output beam from a 308 nm (XeCl) excimer is reshaped in a beam homogenizer and
passed through a photomask (typically chrome on quartz) containing the pattern for one or even several circuits.
The mask is re-imaged at the work surface which consists of a plastic film or web on which a thin layer of metal
has been vapor-deposited. Most of the UV radiation passes through the film and is strongly absorbed at the
plastic-metal interface. This vaporizes a thin layer of the plastic, completely removing the overlying metal film
(see figure 6). Providing the metal layer thickness is 150 nanometers or less, a single laser pulse performs a
complete lift with clean edges and no breaks – even on lines as narrow as 10 microns.
The optimum thickness is actually around 500 angstroms which is more than sufficient for most flex circuit
applications, which typically do not carry high
current. At this thickness, a circuit with area
up to 400 mm2 can be processed at a pulse
energy of 1 J.
Excimer lasers designed for this application
typically operate at pulse repetition rates of
several hundred Hz. At 300 Hz for example,
this “single pulse” laser process can generate
18,000 circuits/minute. The pro-cess can be set
up as reel-to-reel with continuous feed because
the short pulse of the laser eliminates the
possibility of blur even at feedrates of tens of
meters/second.         Alternatively       some
manufacturers have implemented a roll-to-roll
process in which optics sweep across the web
which undergoes stepped motion. Laser direct        Fig.6 Laser Direct Patterning process (LPKF AG)                    Formatted: Font: 10 pt, Bold, Italic
patterning can be used with several different                                                                          Formatted: Font: 10 pt, Italic
flexible plastic substrates (PET, polyimide, PEN, and PMMA) and a full range of conductors including copper,           Formatted: Font: 10 pt, Italic
gold, silver, platinum, aluminum, and even titanium. Manufacturers cite several process advantages, compared to
traditional lithography using wet photochemistry. The most important is process simplicity; a single dry process       Formatted: Font: 10 pt, Italic
replaces about seven separate steps. It also eliminates the cost and disposal of the chemical reagents. In addition,   Formatted: Font: 10 pt, Italic
the metal debris can be trapped by a vacuum system, allowing recycling of this valuable material.




                                                5.     Conclusion

Intelligent thin film development and rapid prototyping for various fields of applications is largely facilitated by
means of short excimer laser wavelengths. Combined with compact, automated vacuum systems for fast and
convenient substrate handling stoichiometric multi-layer thin-films with good homogeneity and taylored physical
characteristics are efficiently generated. Stable, high pulse energy output characteristics provide controlled and
reproducible target ablation for nanotechnology which can often be upscaled in output rate by reel to reel
approaches.



                                                6.     References

[1]   Delmdahl, R F.; Oldershausen, G.:
      Quantitative solid sample analysis by ArF excimer laser ablation, Journal of Molecular Structure,
      Vol. 744, p. 255-258 (2005).
[2]   Ashfold, M. N. R.; Claeyssens, F.; Fuge, G. M.; Henley, S. J.:
      Pulsed laser ablation and deposition of thin films, Chemical Society Review, Vol. 33, p. 23-31 (2003).
[3]   Pedarnig, J. D.; Peruzzi, M.; Vrejoiu, I.; Matei, D. G.; Dinescu, M.; Bäuerle, D.: Pulsed-laser deposition
      of inclined ZnO, of GaPO4 and of novel composite thin films, Applied Physics A, Vol. 81, p. 339-343
      (2005).

				
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